The present invention is related to well drilling fluid additives, and more specifically, to an improved material additive for increasing viscosity and controlling filtration in well drilling and work-over fluids and method of using same.
Oil and gas well drilling and work-over operations usually require the use of a heavy fluid, such as drilling mud or salt water brine, to control reservoir pressure and to carry drill bit cuttings and debris out of the well bore. The hydrostatic head of the fluid in the well must be sufficient to offset the reservoir pressure to prevent blow outs. Drilling mud is usually a fresh water or salt water base fluid. The most common principal ingredient in drilling mud for adding weight to the fluid to increase the hydrostatic head is barite or barium sulfate, which is a naturally occurring insoluble mineral.
In order to keep the barite evenly dispersed in suspension in the drilling fluid, as well as to carry drill bit cuttings and debris out of the well bore, it is necessary to increase the viscosity of the drilling fluid. Two of the most common viscosifiers used in drilling fluids are clays, such as bentonite or attapulgite, which flocculate or expand in water, or starch, which gelatinizes when heated in water. Starches can also be pre-gelatinized, as described in U.S. Pat. No. 2,417,307, issued to D. Larson. Corn starch is the most common starch product used because of its availability, although it is known that starches derived from other cereal grains and vegetables can also be used, as described in the D. Larson patent.
While starches are generally readily available and inexpensive, there are a number of problems associated with the use of conventional starch as a drilling fluid viscosifier agent. For example, starches are comprised of distinct granules which swell when heated to result in the desired increase in viscosity. As temperature continues to increase, however, the swelled granules eventually start to break down. This retrogradation of the starch causes it to turn into a mush with significantly reduced viscosity. Therefore, temperature control is a significant requirement in maintaining the viscosity effectiveness of starch as a drilling fluid viscosifier, yet there is very little temperature control available when drilling into underground formations.
Starch is also very vulnerable to consumption by organic organisms, which can also result in significant loss of viscosity. Since such organisms multiply very rapidly, a drilling fluid viscosified by starch can lose its viscosity very quickly. Although this problem can be controlled by adding preservatives, such as formaldehyde, to the drilling fluid, it must be monitored vigilantly.
Another problem encountered in the use of starch, especially pre-gelatinized starch, as a drilling fluid additive, is that it coagulates very rapidly upon contact with water. This action results in many large and small globules of unhydrated starch surrounded by a layer of water impermeable coagulated starch, which are almost impossible to break down and which prevent the starch inside these globules from effective use. Even mechanical mixers and agitators are not totally effective to break down these globules and disperse the starch uniformly throughout the drilling fluid medium. Consequently, approximately twenty percent (20%) of the starch added to drilling fluid is not hydrated and is not effectively utilized as a viscosifier. Such inefficiency, of course, increases cost and is undesirable.
Another problem that must be controlled in drilling fluids is the loss of water from the drilling fluid matrix into porous ground formations or reservoirs during drilling or well work-over operations. A rapid loss of water into a formation can decrease the hydrostatic head and put the well in jeopardy of a blow out. Also, introduction of water, especially fresh water, into some reservoir formations can cause water sensitive native clays in the reservoir to swell and inhibit flow of hydrocarbons into the well bore. Further, although not as serious, a significant loss of water from the drilling fluid would require a large water source and excessive water hauling and pumping, labor, and equipment to keep the well bore full and the hydrostatic head up to a safe level.
Generally, starch has been at least somewhat effective in controlling filtration, as well as viscosifying the drilling fluid, whereas the clays used for viscosifiers, such as bentonite and attapulgite, are not very effective in filtration control. When clays are used as viscosifying agents, some starch or other filtration control agent must be added to prevent excessive water loss from the drilling fluid matrix.
In recent years, some effective synthetic polymers and floculents have been developed to control filtration when used in combination with the clay viscosifying agents. In some situations, these synthetic polymer additives are superior to the starch products that in the past were commonly used. For example, such new synthetic polymers are not as sensitive to temperature ranges and organic organisms, such as bacteria, which can cause starch products to lose viscosity and filtration control properties. Consequently, the use of starch as a viscosifying and filtration control agent has been replaced in some well drilling operations by the combination of bentonite or attapulgite clays with synthetic polymer filtration control agents, such as "Drispak.TM.," manufactured and sold by Drilling Specialties.
There are also some problems associated with the use of clays and synthetic polymers in drilling fluids. For example, the synthetic polymers are far more expensive than starch products. Also, it is often undesirable to expose some reservoir formations to bentonite or attapulgite clay solids which can permanently clog or plug the formation around the well bore, thus inhibiting the flow of hydrocarbons from the reservoir formation into the well bore. Further, some formations already contain expandable native clays which tend to swell and clog the formation around the well bore upon being exposed to water, particularly fresh water. When such reservoirs are encountered, it is desirable to use a salt water base mud, rather than a fresh water base mud, to minimize damage to the formation. There are also drilling locations where salt water is abundant, but adequate quantities of fresh water are not readily available, so it is desirable to use salt water based drilling fluid for this reason also.
The clay viscosifiers, however, do not expand or flocculate as effectively in salt water as they do in fresh water. It is not unusual for clays to require as long as twelve hours or more to flocculate or "yield" after they have been added to a salt water drilling system which decreases the drilling fluid system efficiency and can increase the cost significantly. It is undesirable and excessively expensive to shut down drilling operations for such a long period of time while waiting for clay additives to flocculate or yield, and it is also an undesirable expense to provide premixing and agitating tanks of sufficient volume at the well site to flocculate the clays prior to adding them to the drilling fluid system.
It is equally undesirable, however, to simply add the unflocculated clay to the drilling fluid medium during drilling operations without waiting a sufficient time for flocculation or yield. Such unflocculated clay particles can enter into interstices in an oil or gas bearing formation relatively easily and then subsequently flocculate or expand to clog the formation and inhibit production of oil or gas therefrom. Also, it is usually necessary to increase viscosity during drill stem testing of a formtion in order to hold suspended solids in the drillling fluid matrix while the drilling fluid is not being circulated. A wait of twelve hours or more for the added clays to flocculate or yield to increase the viscosity is undesirable. Further, when decreased viscosity is desired, such as after drill stem testing, it is difficult to control filtration while decreasing viscosity when clays are used for a viscosifier because the clays have no capacity to control filtration. Another undesirable effect of the use of clays in a drilling fluid system is that clays are abrasive and cause excessive wear in mechanically moving parts, such as in pumps, valves, and the like.
Therefore, there remains a need for an inexpensive method and material for viscosifying and controlling filtration in drilling fluids that is easy to use and more stable and efficient than conventional starch in varying temperatures in environmental conditions, yet more effective in salt water based drilling fluid than clays and synthetic polymers and not damaging to reservoir formations.